The present invention generally relates to vehicle suspensions. More particularly, the present invention relates to vehicle suspensions having springs. Single spring rate suspensions and variable spring rate suspensions for use in vocational or heavy haul truck applications are known. Single spring rate suspensions have a fixed spring rate that generally must be set at a level that produces a suspension with either a comfortable ride or a stiff suspension exhibiting adequate roll stability. As a result, either roll stability or ride quality is compromised in single spring rate suspensions, depending upon the selected spring rate.
Variable spring rate suspensions overcome this deficiency of single spring rate suspensions by providing for multiple spring rates during operation. As the sprung load is increased, the spring rate is correspondingly increased. An example of a variable spring rate elastomeric spring suspension for use in vocational or heavy haul truck applications is shown in U.S. Pat. No. 6,585,286, the disclosure of which is hereby incorporated herein by reference. That suspension utilizes bolster springs and auxiliary springs to achieve its variable spring rate.
The assignee of the present invention disclosed a vehicle suspension having shear springs and a load cushion with a continuously increasing spring rate in U.S. application Ser. No. 12/876,158 which is entitled “Suspension Assembly With Tie-Plate” and was filed on Sep. 5, 2010, which is a continuation-in-part of U.S. patent application Ser. No. 12/545,828 which is entitled “Tie-plate and frame hanger of a suspension assembly” and was filed Aug. 22, 2009, which is a continuation-in-part of U.S. patent application Ser. No. 12/334,195, entitled “Modular Suspension System and Components Thereof” filed on Dec. 12, 2008, and a continuation-in-part of U.S. patent application Ser. No. 12/045,069, entitled “Elastomeric Spring Vehicle Suspension” filed on Mar. 10, 2008, now U.S. Pat. No. 7,926,836, each of which is assigned to Hendrickson USA, L.L.C. This application incorporates U.S. patent application Ser. Nos. 12/545,828, 12/334,195, and 12,876,158, and U.S. Pat. No. 7,926,836 herein by reference. The present application includes improvements and advancements over the vehicle suspensions disclosed in the applications noted above.